Australia now world's largest exporter of Natural Gas

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At what temperature are you basing your statement on?

There is a Critical Pressure, and a Critical Temperature.

I looked it up - waiting for a scrip fill - Tcrit is 190.55°K, so not practical on this planet but is on a high gravity gas giant.

But I perceive the post to be the recovery compressor injects that previously lower pressure gas back with the main feed gas to the liquefaction facility. Quite practical.
 
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I'm sorry, I really don't understand what you're trying to say. Yes that is the critical point temperature for methane. But that is easily achievable, I'm not sure what the reference to a gas giant has to do with it. If the temperature is not below that then methane will not liquefy regardless of the pressure.

The critical point temperature is where it would be transported at, unless the pressure is increased. But as far as I know it is transported at near atmospheric pressure.
 
The terminal could have a subsea vent line feeding a riser to flare … but I'd be surprised if the tanker burned any gas …

The risk would be very high … and when not tied up the ship could control the bow heading to vent per wind direction in an emergency …
 
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Originally Posted by kschachn
Originally Posted by badtlc
Not many flare it. Most LNG facilities recompress what evaporates.

Recompress? That won't re-liquefy it.


Compressing is how you liquify it.
 
Originally Posted by kschachn
Originally Posted by Nyogtha
Well, eventually it can. But what badtlc is speaking of, it's recovered and put in with the main gas production for treatment and then to market. Whether that later gets liquified or not just depends.

Part of getting natural gas to liquid form involves compression as well, to get it to a combination of high pressure and low temperature that results in liquid form.

No, not without cooling as you note. The phase diagram shows it won't liquefy due to pressure alone unless the temperature is low enough.

LNG transport ships operate at near atmospheric pressure, correct?


What evaporates is already cold. It is barely above the liquid temp and just needs compressed. That alone removes enough heat to liquify. You aren't compressing room temp gases.
 
Originally Posted by badtlc
What evaporates is already cold. It is barely above the liquid temp and just needs compressed. That alone removes enough heat to liquify. You aren't compressing room temp gases.

And then what, allowed to expand? It would be heated by the compression cycle as well.
 
LNG process/shipping requires refrigeration … hence my comment about the giant thermos bottles (tankers) …
 
Originally Posted by kschachn
Originally Posted by badtlc
What evaporates is already cold. It is barely above the liquid temp and just needs compressed. That alone removes enough heat to liquify. You aren't compressing room temp gases.

And then what, allowed to expand? It would be heated by the compression cycle as well.


Letting some boil off is how they keep it cool...they capture that, and at the plant recompress it, or on board send it to the engines (in some cases).
 
Known as autorefrigeration. I see the "Boil Off Compressors" in the simplified diagram posted by 4WD.

Kinda like the way an air conditioner or refrigerator works, except those are closed loop processes, hence not "auto".

Hence the need to vent on the ships to maintain liquid state - via open loop autorefrigeration. I can especially see this when the main engines are shut down at end of sea passage and the ships are waiting for dock availability and local pilots to take them from the point of end of sea passage to wherever they need to dock.

But at the liquefaction plant simply recompress. It is reliquified at the boiloff gas condensers. True adiabatic compresdion won't reliquefy due to heat of compression, no more than a refrigerator or air conditioner (both have condensers), but I didn't realize you were posting from a true adiabatic perspective instead of a working process perspective kschachn.

No continuous ambient temperatures of 190°K or below where natural gas is produced on this planet is my point, to be used as a heat sink, but liquid methane is known to exist on the high gravity gas giants. No refrigeration facilities detected on those bodies yet AFAIK.

https://solarsystem.nasa.gov/resources/677/gas-giant-interiors-2003/

Of course things get more complicated if any appreciable water is present as then you get methane hydrates, which are in solid crystalline form along the sea floor at the pressures at those water depths.

Does this help understand what I posted kschachn?
 
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If it just boil off in the big tank, it is still low pressure / temperature, and shouldn't take significantly more energy to compress / cool it back to liquid and inject back into the tank. Of course you will need some energy to run the refrigeration / compression, so it make sense to use some of the gas to run the compressor and probably even better to submerge the condenser in the ocean (may cost a lot due to condenser corrosion, but it may be enough energy savings doing so).

It should use much less energy to do it as soon as it boil off than the initial compression before you pump it into the ship.
 
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